1. Technical Field
The invention herein generally relates to rechargeable lithium ion batteries and more particularly, to batteries having electrodes made from Fe-substituted LiCoPO4.
2. Description of the Related Art
Electrochemical lithiated metal phosphate materials are finding increasing utility as components of electrodes for electrochemical devices, and in particular, as components of cathodes for rechargeable lithium-ion batteries. In the operation of such batteries, lithium ions are transferred, via an appropriate electrolyte, from the positive electrode (cathode) to the negative electrode (anode) during charging and from the anode to the cathode during discharge.
Lithiated metal phosphates have shown good thermal stability, low reactivity with electrolytes and have very good lithium transport and storage properties which allow for the manufacture of lithium ion batteries having large charge storage capability. Lithiated metal phosphates of the formula LiMPO4, where M=Fe, Mn, Co or Ni, have been of strong interest for charge storage. See Pandi et al, J. Electrochem. Soc., Vol. 144, 1188-1194 (1997). The voltage of the electrochemical cell varies with M from 3.4 V for Fe, 4.1 V for Mn, 4.8 V for Co and 5.1 V for Ni. High voltage batteries, that could be obtained with LiCoPO4, for example, are desirable because the stored energy is proportional to the voltage and the power is proportional to the square of the voltage. However, these higher voltage electrode materials and LiCoPO4, in particular, have shown poor charge/discharge cycle life and relatively low electronic conductivity.
The prior art has implemented various approaches which have enhanced the electronic conductivity of these materials, such as coatings with conductive materials, synthesis under a reductive atmosphere and ball milling with conductive materials. However up until now, these electrode materials have demonstrated a poor cycle life.